The invention concerns a blanket for a printing machine with variable surface properties.
The blanket of the invention is more particularly designed for machines permitting putting into operation a printing process in a indirect manner, that is to say, with an intermediate transfer element. One can namely point to machines which permit putting into operation an offset, indirect heliography type process, or an indirect digital printing process with solid or liquid toner.
First of all, printing color images is generally obtained by superposition of at least four primary colors (cyan, magenta, yellow and black). These four primary colors are successively printed in printing groups distant from one another. This distance between groups can generate deviations for the trajectory of the band of paper to be printed as well as deformations in this band. Thus, deviations in positioning the various primary colors affect the quality of the printed image.
On the rotary printing presses serving to print newspapers, the phenomenon called xe2x80x9cfan-outxe2x80x9d appears which manifests itself by stretching the paper between different printing groups and therefore by a loss of registers for the four color printers.
On sleeve presses which are wider than the rotary printing presses of the older generation, one likewise notes phenomena of broadening or shrinkage of the dimensions effectively printed by each group. These phenomena have as a result a loss of lateral register on the edge of the printed span.
These printing errors become even more visible with an increase in the width of the printing machine and the printing speed.
One can likewise note inconveniences more particularly associated with the offset printing process.
In offset printing, the printing cylinders are lined with a blanket which ink borne by an offset plate, itself borne by a cylinder which was previously covered by a film of ink and water. The blanket-bearing cylinder conducts the printing on a band of paper, for example, which is held by a counter pressure cylinder.
Thus, the offset printing process is characterized by the presence of moistening water and by the high viscosity of the inks.
Upon leaving printing, that it at the level of the zone of contact between the blanket-bearing cylinder and the counter pressure cylinder (commonly called the xe2x80x9cprinting nip zonexe2x80x9d), high separation stresses are generated which induce a poor relaxation of the paper by reason of an excessive relative adherence between the ink and the surface of the blanket.
This traction exerted on the paper has the tendency, on wide machines, to deform it in the shape of a paper hat which induces register defects.
Moreover, from the fact of this relatively significant adherence, fibers can be detached from the band of paper and will accumulate on the blanket itself, which is of a nature to cause printing quality to deteriorate progressively.
Otherwise, the moistening water assures the segregation of printing zones and no-printing zones and allows balancing the offset printing process. Nonetheless, the presence of water can deform the paper or at least alter its mechanical characteristics. This can likewise entail register defects between the different printing groups.
Some solutions have already been proposed within the state of the art to resolve these problems.
One can in particular cite the document EP-0 659 585 which describes a blanket presenting, by its construction, a concave or convex profile. This profile is designed to eliminate deviations in surface scrolling speed on the direction across the band of paper to avoid thus the appearance of possible folds.
Nevertheless, such a blanket leads to the appearance of different pressures in the direction of the width of the blanket which can cause printing defects, namely a diminution of printing contrast values.
One can likewise cite document WO 95/23706 which describes compressible layer blankets presenting a central region of greater compressibility than the peripheral parts.
For this, the compressible layer can present a parabolic profile, the variation in thickness of this layer leading to a variation in compressibility.
Such a blanket has for its object guaranteeing a better holding of the registers.
Nevertheless, it has been possible to confirm that the sleeves proposed by the state of the art do not permit complete resolution of the problem of printing defects.
In fact, the solutions proposed by the state of the art only take into consideration the mechanical characteristics of the printing process, such as the pressure in the contact zone between the two cylinders or the width of the cylinder.
Thus, a practically non-measurable deformation, for example, a deformation of 0.005% of the paper on a span of 2000 mm suffices to create a lateral register defect of 50 xcexcm at the extremities. This defect is redhibitory in terms of quality.
Within the framework of the invention, it has been shown that the surface properties of the blankets have a significant influence on the quality of transfer during printing which specifically depends upon printing clearance, the quality of the flat tints, the relaxation of the paper, the transfer of moistening water the paper, paper flow or even the holding of the registers.
Thus, the surface properties of the blanket exert an influence on the separation of the ink film specifically upon leaving the printing nip while more or less facilitating the relaxation of the band of paper. The surface properties thus exert an influence on the deformation of the entirety of the band of paper if these surface properties are variable in a crosswise direction.
Likewise, a variation of the surface properties of the blanket can induce a variation in the quantity of moistening water transmitted to the paper and thus modify the level of mechanical deformation of the entirety of this during the printing process.
The invention therefore has as its purpose palliating the inconveniences known in the state of the art by proposing a printing blanket permitting optimizing the quality of printing taking into consideration the surface properties of the blanket.
Thus, the invention relates to a printing blanket for a printing machine cylinder, specifically of offset type, including a rectified or polished, or even molded lithographic layer, at least one reinforcing or support layer and at least one compressible layer, characterized in that at least one surface property of the blanket, namely the overall surface energy of the blanket, the surface energy with polar character or the mean roughness of the blanket, varies on the outer surface of the blanket in a crosswise direction.
The definition of surface energy with a polar character constitutes at the same time indiscriminately a means for measuring acid-basic type interactions and amphoteric type interactions of the surface of the blanket with the moistening water and the ink.
Preferably, this surface property is constant in the circumferential direction of the blanket.
Otherwise, this surface property of the blanket varies advantageously in a symmetrical manner in relation to the median transversal plane of the blanket.
By way of example, this surface property of the blanket varies from one edge of the blanket to a transversal plane, and in the transverse direction in a discontinuous manner, specifically by discrete increments, or continuously, namely in a linear manner or according to a parabolic, cubic or sigmoid type of profile.
Moreover, this surface property of the blanket can increase or decrease from one edge of the blanket to the transversal plane in the transverse direction.
In one embodiment of the printing blanket of the invention, when a blanket is installed on the cylinder of the printing machine, its outer surface is perceptible cylindrical.
In another embodiment, when the blanket is installed on the cylinder of the printing machine, the outer surface of the blanket presents a concave or convex contour.
Moreover, the compressible layer of the printing blanket of the invention advantageously has a variable compressibility in the crosswise direction.
Preferably, the overall surface energy of the printing blanket of the invention varies between two extreme boundaries each falling between 5 and 50 mJ/m2, and advantageously between 10 and 30 mJ/m2.
Likewise preferably, the surface energy with a polar character of the printing blanket of the invention varies between two extreme boundaries each comprised between 0 and 25 mJ/m2.
This polar component of surface energy measures an acidic, basic or amphoteric character of the surface of the blanket on a case by case basis.
Finally, the average roughness of the printing blanket of the invention preferably between two extreme limits each falling between 0.3 xcexcm and 2 xcexcm.
The invention likewise concerns a process for creating a printing blanket according to the invention.
This process consists in photochemically grafting some monomers on the outer surface of a blanket including a ground or polished or even molded lithographic layer, at least one reinforcement or support layer, and at least one compressible layer, the density of the grafting being variable according to the crosswise direction of the blanket such that at least one surface property of the blanket, namely the overall surface energy or the surface energy with a polar character of the blanket, likewise varies in the cross wise direction.
The process of the invention can likewise consist of conducting, on the outer surface of a blanket including a ground or polished or even molded lithographic layer, at least one reinforcement or support layer and at least one compressible layer, a chemical treatment, by ionization or even by heating, specifically of flame type, this process being conducted in a variable manner in the crosswise direction of the blanket such that at least one surface property of the blanket, specifically the overall surface energy of the blanket, likewise varies in the crosswise direction.
The process of the invention can even consist of mechanical processing, for example, abrading or machining, especially by laser, of the outer surface of a blanket including a ground or polished, or even molded lithographic layer, at least one reinforcement or support layer, and at least one compressible layer, this treatment being conducted in a variable manner in the crosswise direction of the blanket such that at least one property of the blanket, namely its average roughness, likewise varies crosswise.
The process of the invention likewise consists of manufacturing a blanket including a lithographic layer, at least one reinforcement or support layer and at least one compressible layer, the lithographic layer being obtained by molding and the outer surface of the blanket presenting an average roughness which is variable in the crosswise direction of the blanket, obtained directly by molding.